Optical device

ABSTRACT

A photonic crystal  2  with plasticity is arranged by making microspheres of silica or barium titanate or air bubbles be contained in a gel substance. When an external force is applied to this photonic crystal, photonic crystal  2  deforms and the photonic band gap is readily changed thereby. When the photonic band gap changes, the passage of light of a specific wavelength is restricted. Light of a desired wavelength is thus output from photonic crystal  2 . With this invention, this wavelength can be varied readily by means of an external force.

TECHNICAL FIELD

This invention relates to an optical device using a photonic crystal.

BACKGROUND ART

A semiconductor monocrystal is a substance with which specific atoms arealigned in a periodic and regular manner. Its electron propagationcharacteristics are determined by the atomic interval inside thesemiconductor crystal. That is, a semiconductor has an energy band gap,and this energy band gap is determined by the wave properties ofelectrons and the periodic potential of the atoms.

Meanwhile, a photonic crystal is a three-dimensional structure whereinsubstances that exhibit a potential difference with respect to light, inother words, substances with a refractive index difference are alignedin a period close to the wavelength of light. Such substances that makeup a photonic crystal have been proposed by Yablonovic and others.

Within a photonic crystal, the optical propagation characteristics arelimited by the constraints of the wave properties of light. That is, thepropagation of light inside a photonic crystal are subject torestrictions in a manner similar to the propagation of electrons in asemiconductor. In a photonic crystal, a forbidden zone for light orso-called photonic band gap exists, and due to the existence of thisband gap, light of a specific wavelength band cannot propagate insidethe crystal.

Various photonic crystals have been proposed since priorly. For example,there are photonic crystals wherein submicron particles are aligned in aperiod close to the wavelength of light. For microwave bands, there arephotonic crystals in which polymer spheres are aligned within space asthe particles.

Besides these, there are photonic crystals with which polymer spheresare hardened inside a metal and thereafter the polymer spheres aredissolved chemically to form periodic microscopic spaces inside themetal, photonic crystals with which holes are bored at equal intervalswithin a metal, photonic crystals, with which regions that differ fromtheir surroundings in refractive index are formed in a solid materialusing a laser, photonic crystals, with which a photopolymerizing polymeris processed to a groove-like form using a lithography technique, etc. Aphotonic crystal that has been formed by such processing has a photonicband gap that is uniquely determined by the structure.

An optical device using such a photonic crystal can selectively output apredetermined wavelength range of input light. In the description thatfollows, light that is input into a photonic crystal shall be referredto as “input light” and light that is output from a photonic crystalupon passage through the photonic crystal shall be referred to as“output light.”

DISCLOSURE OF THE INVENTION

However, with an optical device, since the photonic band gap of aphotonic crystal cannot be varied adequately, the wavelength of theoutput light cannot be varied. This invention has been made in view ofthis problem, and an object thereof is to provide an optical device,with which the wavelength of the output light can be varied adequatelyby means of deformation by external force.

In order to resolve the above problem, this invention provides in anoptical device, with which an external force is applied to a photoniccrystal to change the photonic band gap of the photonic crystal, anoptical device with which the photonic crystal has plasticity.

Since the photonic crystal of this invention has plasticity, when anexternal force is applied and the photonic crystal is deformed, thephotonic band gap changes greatly and the wavelength of the output lightfrom the photonic crystal changes adequately. With such an opticaldevice, since effective wavelength selection can be performed even ifthe volume of the photonic crystal itself is made small, the entiredevice can be made compact.

This invention's optical device is characterized in furthermorecomprising an external force application means for applying theabovementioned external force. As the external force application means,various types are possible.

One type of arrangement is characterized in that the external forceapplication means is a piezoelectric element that deforms in accordancewith an electrical input. In this case, since an external force isapplied to the photonic crystal by means of deformation of thepiezoelectric element by an electrical input, a system that performs anelectrical input based on a specific measured value, etc., can bearranged.

Another type of arrangement is characterized in that the external forceapplication means is a pressing mechanism that presses the photoniccrystal in accordance with a manual input. In this case, since fineadjustment of a manually applied external force is enabled in anexperimental measurement system, the device can be applied to basicresearch on photonic crystals, etc.

Yet another type of arrangement is characterized in that the externalforce application means is a hollow member, which can be deformed inmanner such that its inner diameter changes, and the photonic crystal isdisposed inside the hollow member. Since the hollow member deforms in amanner such that its inner diameter changes, the photonic crystaldeforms in accordance with this deformation in a manner such that itexpands or contracts in the length direction of the hollow member. Theinput light is input from one end in the length direction of thephotonic crystal and is output from the other end. The spreading oflight in the radial direction can thus be restrained and the lowering ofintensity per unit area of the output light can be restrained.

Also, an optical device by this invention is characterized infurthermore equipping a feedback means, which measures a physicalquantity that varies in accordance with the photonic band gap of thephotonic crystal and controls the magnitude of the external forceapplied by the external force application means in accordance with themeasured value. In order to obtain a desired photonic band gap, aphysical quantity, which varies in accordance with the photonic band gapand is preferably the output light intensity or output light spectrum,is measured and the feedback means is made to control the external forceapplication means so that the intensity of output light or the intensityof a specific wavelength will be constant.

Also, an optical device by this invention is characterized infurthermore equipping a heater, which heats the photonic crystal, and atemperature sensor, which measures the temperature of the photoniccrystal and in that the power supplied to the heater is controlled inaccordance with the temperature measured by the temperature sensor. Inthis case, since heating is performed while measuring the temperature ofthe photonic crystal with the temperature sensor, the temperature of thephotonic crystal can be set to a desired value, which is preferably afixed value, in order to restrict variation of the photonic band gap dueto temperature.

Also, an optical device by this invention is characterized infurthermore equipping a container, which houses the photonic crystal,and in that the external force application means applies pressure in afixed direction as the abovementioned external force to the photoniccrystal that is housed inside the container. In this case, the photoniccrystal may be held by an outer wall of the container to restrictdeformation due to forces besides the desired external force and limitthe direction of deformation.

Also, an optical device by this invention is characterized in that atleast a part of the outer walls of the container is transparent or atransparent window is disposed at this part and light is input into thephotonic crystal via this part. In this case, the input light is inputinto the photonic crystal via the transparent outer wall or window, andsince the photonic crystal is held by the corresponding outer wall, thenumber of parts can be reduced.

The abovementioned photonic crystal is characterized in that a pluralityof microspheres of silica or barium titanate are contained in a gelsubstance. The abovementioned photonic crystal may also be arranged tocontain a plurality of microscopic spaces formed inside a gel substance.In such cases, the photonic crystal can be deformed readily.

Also, an optical device by this invention is characterized in that theabovementioned container is formed by processing a semiconductorsubstrate and the piezoelectric element is formed on the semiconductorsubstrate. In this case, since the photonic crystal is disposed in thecontainer, in particular, in an indented part formed on thesemiconductor substrate and the piezoelectric element is formed on thissemiconductor substrate, these components can be formed usingsemiconductor microelectromechanic techniques and the entire device canbe made compact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of an optical device of an embodiment.

FIG. 2 is a perspective view of a photonic crystal 2.

FIG. 3 is a graph, showing the wavelength (nm) dependence of thetransmittance (arbitrary constant) of output light by a dichroic mirror.

FIG. 4 is an explanatory diagram, showing a favorable example of anoptical device.

FIGS. 5A, 5B, and 5C are graphs, showing the wavelength (nm) dependenceof the transmittance (arbitrary constant) of output light by a dichroicmirror.

FIG. 6 is a perspective view of a photonic crystal 2 that uses airbubbles.

FIG. 7 is an explanatory diagram of an optical device of anotherembodiment.

FIG. 8 is an explanatory diagram of an optical device of anotherembodiment.

FIG. 9 is a perspective view, showing the principal parts of an opticaldevice that uses a tube-type piezoelectric element.

FIG. 10 is an explanatory diagram of an optical device of yet anotherembodiment.

FIG. 11 is an explanatory diagram of an optical device of yet anotherembodiment.

FIG. 12 is an explanatory diagram of an optical device of yet anotherembodiment.

BEST MODES FOR CARRYING OUT THE INVENTION

Optical devices of embodiments shall now be described. Elements that arethe same or have the same functions shall be provided with the samesymbols and redundant descriptions shall be omitted.

FIG. 1 is an explanatory diagram of an optical device of an embodiment.This optical device is a device that selects a desired wavelength bandfrom the wavelength band of input light and outputs light of thisdesired wavelength band as output light. A photonic crystal 2 is placedon a base 1, and photonic crystal 2 is urged by apressurizing/depressurizing device (external force application means) 3,which applies pressure or reduces the pressure applied to photoniccrystal 2.

Photonic crystal 2 deforms in a precise manner in accordance with theapplication of an external force and is a substance with which thephotonic band gap changes in accordance with the deformation. Whenphotonic crystal 2 is deformed by pressurizing/depressurizing device 3,its photonic band gap changes. Pressurizing/depressurizing device 3 iscontrolled by an external pressure control device (external forcecontrol means) 4, and external pressure control device 4 controls themagnitude and duration of application of the abovementioned externalforce.

The input light is input into photonic crystal 2 upon passage through afirst optical element 5 that allows propagation of light. Components ofspecific wavelengths in the input light cannot pass through photoniccrystal 2 and a predetermined wavelength band is selected in accordancewith the photonic band gap (optical response characteristic) and outputas the output light from photonic crystal 2. The output light is inputinto a second optical element 6 that allows propagation of light and isoutput to the exterior of the present optical device via second opticalelement 6. That is, the optical coupling characteristic between firstand second optical elements 5 and 6 is changed by the application ofexternal force.

The present optical device is an optical device, with which the photonicband gap of photonic crystal 2 is changed by the application of externalforce to photonic crystal 2, and photonic crystal 2 has plasticity.Photonic crystal 2 may also have elasticity.

Since photonic crystal 2 has plasticity, when photonic crystal 2 isdeformed by the application of external force, the photonic band gapchanges greatly and the wavelength of the output light from photoniccrystal 2 thus changes adequately. With such an optical device, sinceeffective wavelength selection will be enabled even when the volume ofphotonic crystal 2 itself is made small, it is possible to make theentire device compact.

FIG. 2 is a perspective view of photonic crystal 2.

With this photonic crystal 2, a plurality of microspheres (opticalmicrocrystals) 2B of silica or barium titanate are contained inside agel substance 2G. This photonic crystal 2 can be deformed readily.Microspheres 2B are aligned uniformly in a regular manner at a periodclose to the wavelength of light inside substance 2G. The intervalbetween microspheres 2B is half to one-fourth the wavelength of thelight that is to be selected, and microspheres 2B are transparent tothis wavelength. When light of a wavelength band Δλ (which includes λ₁)is made to enter photonic crystal 2, only components of a specificwavelength band λ₁ are transmitted through photonic crystal 2 inaccordance with the photonic band gap.

Since a gel is readily deformed by external force, the photonic band gapof photonic crystal 2 changes readily. Due to this change, theabovementioned wavelength band λ₁ that passes through photonic crystal 2changes. Microspheres 2B and substance 2G differ in refractive index andboth are transparent to the selected wavelength of light.

For example, a material having an ultraviolet-curing resin mixed thereinmay be used as a sol material and gelling may be accomplished byilluminating this material with ultraviolet rays. A mixture of acrosslinking agent and a photopolymerization initiator in acrylamide isa representative example of an ultraviolet-curing resin, and variousother examples are known since priorly.

Since it is sufficient for the number of periodic structures ofmicrospheres 2B to be approximately 50, photonic crystal 2 will functionadequately even if it is an element with a maximum size of 100 μmsquare. Compactness of a device can thus be realized by using thisphotonic crystal 2.

FIG. 3 is a graph, showing the wavelength (nm) dependence of thetransmittance (arbitrary constant) of output light by a photonic crystalwith a multilayer film structure, that is, a dichroic mirror. The inputlight is white light. Though this graph is not that of theabove-described photonic crystal 2, in a case where microspheres 2B arealigned at completely equal intervals, the optical characteristics for aspecific direction will be the same as those illustrated in this Figure.With the present example, the transmittance of light near the wavelengthof 400 nm is lowered in comparison to light of the surroundingwavelength bands.

The above-described optical device has external force application means3 for applying the abovementioned external force, and various types ofexternal force application means are possible.

FIG. 4 is an explanatory diagram, showing a favorable example of theabove-described optical device. In this case, the above-describedexternal force application means 3 is a piezoelectric element (piezoelement) 3′, which deforms in accordance with an electrical input. Avoltage variable power supply 4′ is used as external pressure controldevice 4.

With the present example, piezoelectric element 3′ moves in thedirection perpendicular to the surface of base 1 in accordance with thevoltage applied from power supply 4′. A presser plate 3″ is disposedabove photonic crystal 2, and together with base 1, presser plate 3″sandwiches photonic crystal 2. The upper surface of piezoelectricelement 3′ is set at a position that is fixed with respect to base 1 andthe lower surface is fixed to presser plate 3″. Since when piezoelectricelement 3′ expands or contracts, the distance between presser plate 3″and base 1 changes, photonic crystal 2 deforms in an elongating manneralong the optical path.

With such an arrangement, since piezoelectric element 3′ is deformed byan electrical input and applies an external force to photonic crystal 2,a system that performs an electrical input based on a specific measuredvalue, etc., can be arranged.

FIGS. 5A, SB, and 5C are graphs, showing the wavelength (nm) dependenceof the transmittance (arbitrary constant) of output light by a photoniccrystal with a multilayer film structure, that is, a dichroic mirror.FIG. 5A is a graph for the case where an external force is not appliedto the dichroic mirror, FIG. 5B is a graph for the case where a pressureis applied to give rise to a 1% lattice distortion in the directionperpendicular to the mirror, and FIG. 5C is a graph for the case where apressure is applied to give rise to a 1% lattice distortion in thedirection perpendicular to the mirror. A pressure may also be applied togive rise to a lattice distortion a long the mirror surface.

As shown by these graphs, the wavelength λ_(CENTER) at which the peakintensity of the reflectance spectrum lies is approximately 1.5 μm inthe case where there is no external force. The wavelength μ_(CENTER)shifts to approximately 1470 nm (to the shorter wavelength side) when a1% compressive strain is applied and shifts to 1530 nm (to the longerwavelength side) when a 1% expansive strain is applied.

Though the characteristics shown in these graphs are not those of thephotonic crystal 2 shown in FIG. 4, the trends of variation of theoptical characteristics of photonic crystal 2 are the same as thoseshown in these graphs and the wavelength band of the output light varieswith external force, in other words, strain.

The abovementioned photonic crystal 2 may be one with which a pluralityof microscopic spaces are formed and contained in a gel substance.

FIG. 6 is a perspective view of a photonic crystal 2 that uses airbubbles as the abovementioned microscopic spaces. This photonic crystal2 has a plurality of air bubbles 2B′ inside a substance 2G, and the airbubbles 2B′ take the place of the abovementioned microspheres 2B. Such aphotonic crystal 2 can also be deformed readily by an external force.

FIG. 7 is an explanatory diagram of an optical device of anotherembodiment. With this example, a pressing mechanism 3, which pressesphotonic crystal 2 in accordance with a manual input, is arranged as theexternal force application means 3 of FIG. 1, and besides externalpressure control device being operated manually, the arrangement is thesame as that of FIG. 1. Pressing mechanism 3 is equipped with asupporting plate 3 a, which is disposed at a position that is fixed withrespect to base 1, a screw part 3 b, which is engaged with a threadedhole provided in supporting plate 3 a, and a screw feeding mechanism,comprising a screw turning head 3 c that is fixed to one end of screwpart 3 b, and a presser plate 3″ is put in contact with the other end ofscrew part 3 b.

When head 3 c is rotated in a predetermined direction, screw part 3 bmoves in the direction of presser plate 3″. Since photonic crystal 2 isfixed by an adhesive agent to the lower surface of presser plate 3″, ashead 3 c is rotated, an external force is applied to photonic crystal 2and photonic crystal 2 deforms in an elongating manner along the opticalpath. Photonic crystal 2 has plasticity and can also be deformed in acompressive and expansive manner.

With such an optical system, fine adjustment of a manually appliedexternal force is enabled in an experimental measurement system. Thepresent device can thus be applied to basic research, etc., on photoniccrystals.

FIG. 8 is an explanatory diagram of an optical device of anotherembodiment.

This optical device is furthermore equipped with a container 9 thathouses a photonic crystal 2, and an external force application means 3applies pressure as an external force in a fixed direction with respectto photonic crystal 2 housed inside container 9. In this case, photoniccrystal 2 may be held by an outer wall of container 9 to restrictdeformation due to forces besides the desired external force and limitthe direction of deformation. With this example, a piezoelectric element3′ is used as external force application means 3.

At least a part of the outer walls of container 9, that is, the opticalpath for the input light is transparent. Or, a transparent window may bedisposed at this part. The input light is input into photonic crystal 2via the transparent wall or window. The outer wall in the optical pathfor the output light may also be transparent. Since photonic crystal 2is held by the corresponding outer wall, the number of parts required ofthe device can be reduced. A tube-type piezoelectric element may also beused as external force application means 3 for photonic crystal 2.

FIG. 9 is a perspective view, showing the principal parts of an opticaldevice that uses a tube-type piezoelectric element. Piezoelectricelement (hollow member) 3′, which serves as external force applicationmeans 3, is a tube-type element and this is used as a container insidewhich a photonic crystal 2 is disposed. That is, external forceapplication means 3 of this example is a piezoelectric element 3′, whichis deformable in a manner such that its inner diameter changes, and thephotonic crystal is disposed inside piezoelectric element 3′.

Piezoelectric element 3′ deforms in a manner such that its innerdiameter changes, and in accordance with this deformation, photoniccrystal 2 deforms in a manner such that it expands or contracts in thelength direction of the hollow piezoelectric element. The input light isinput from one end in the length direction of photonic crystal 2 and theoutput light is output from the other end. Spreading of light in theradial direction can thus be restrained, and by use of this type ofpiezoelectric element, the lowering of intensity per unit area of theoutput light can be restrained.

FIG. 10 is an explanatory diagram of an optical device of yet anotherembodiment. This optical device differs from that shown in FIG. 4 inthat a window member 10 is adhered onto the light input surface of aphotonic crystal 2, and besides this, the arrangement is the same asthat shown in FIG. 4. The input light that exits from first opticalelement 5 is introduced via window member 10 into photonic crystal 2.With this embodiment, the light input surface of photonic crystal 2 canbe protected by means of window material 10. Window material 10 may alsobe an optical filter.

FIG. 11 is an explanatory diagram of an optical device of yet anotherembodiment. This optical device differs from that shown in FIG. 10 inthat a window member 10 is adhered onto the light output surface of aphotonic crystal 2 as well and in that a feedback means (opticalcharacteristic measurement equipment 11 and power supply 4′), whichmeasures a physical quantity that varies in accordance with the photonicband gap of photonic crystal 2 and controls the magnitude of theexternal force applied by external force application means 3 inaccordance with the measured value, is provided, and besides these, thearrangement is the same as that shown in FIG. 10.

In order to make the photonic band gap of photonic crystal 2 a desiredphotonic band gap, a physical quantity, which varies in accordance withthe photonic band gap and is preferably the output light intensity oroutput light spectrum, is measured and feedback means 11 and 4′ are madeto control external force application means 3 so that the intensity ofoutput light or the intensity of a specific wavelength will be constant.

To be more detailed, the light that has been modulated by photoniccrystal 2 is subject as the output light to measurement of its intensityspectrum by optical characteristic measurement equipment 11 andpiezoelectric element 3′, which serves as a structure control device, issubject to feedback control so that the measured data will take on aspecific value. For example, if the intensity of a specific wavelengththat is measured is low, piezoelectric element 3′ is made to expand orcontract in a predetermined direction and when this causes the intensityto increase, piezoelectric element 3′ is made to expand or contract inthe same direction, while when the intensity decreases, piezoelectricelement 3′ is made to expand or contract in the opposite direction.

By this feedback control, the output light response characteristics ofphotonic crystal 2 can be stabilized and made high in precision.

Photonic crystal 2 may also be produced using semiconductormicroelectromechanic (MEMS) techniques. For example, the abovementionedcontainer is formed by processing a semiconductor substrate andpiezoelectric element 3′ is formed on this semiconductor substrate (notshown). In this case, since photonic crystal 2 is disposed in thecontainer, in particular, in an indented part formed on thesemiconductor substrate and piezoelectric element 3′ is formed on thissemiconductor substrate, semiconductor microelectromechanic techniquescan be used to form these components and make the entire device compact.Needless to say, a driving circuit for piezoelectric element 3′, a powersupply, a photodiode with wavelength filter, etc., may also be formedwithin the semiconductor substrate.

Semiconductor microelectromechanic techniques are also used to prepare,for example, the probe of a scanning tunnel microscope. This probe isprovided with a piezoelectric element and expansion and contraction ofthe piezoelectric element can be controlled in the order of a few nm's.

FIG. 12 is an explanatory diagram of an optical device of yet anotherembodiment. This optical device differs from that shown in FIG. 11 inbeing equipped with a heater 12, a temperature sensor 13, and a heaterpower supply 14, and besides these, the arrangement is the same as thatshown in FIG. 11.

With this optical device, heater 12, which heats a photonic crystal 2,is disposed on a base 1. Temperature sensor 13, which measures thetemperature of photonic crystal 2, is also disposed on base 1. These arepositioned close to photonic crystal 2. Heater power supply 14 controlsthe power supplied to heater 12 in accordance with the temperaturemeasured by temperature sensor 13.

With this embodiment, since photonic crystal 2 is heated while itstemperature is measured by means of temperature sensor 13, thetemperature of photonic crystal 2 can be set to a desired value, whichis preferably a fixed value, to restrain variation of the photonic bandgap due to temperature. Heater 12 and temperature sensor 13 may also beformed in a monolithic manner using MEMS techniques.

A Fabry-Perot interferometer and multilayer film mirror (dichroicmirror) are also 0-dimensional or 1-dimensional photonic crystals.Photonic crystal 2 may be applied to such uses. With a soft photoniccrystal 2, such as described above, it is anticipated that furtherresearch will be carried out on the stability of the sizes and alignmentof microspheres 2B or air bubbles 2B′, mechanical precision forimproving the controllability, long-term stability of the gel,temperature stability, methods of connection with optical fibers andother optical parts, gel-sealing containers, external force applicationmechanisms that can apply the same external force each time, etc.

INDUSTRIAL APPLICABILITY

This invention can be used in an optical device that uses a photoniccrystal.

1. An optical device comprising: a three-dimensional photonic crystalhaving plasticity; and external force application means for applying anexternal force to said three-dimensional photonic crystal to change thephotonic band gap of said photonic crystal in any of three dimensionsthe photonic band gap of said photonic crystal being capable of changingin any of the three dimensions.
 2. The optical device as set forth inclaim 1, furthermore comprising a feedback means, which measures aphysical quantity that varies in accordance with the photonic band gapof the photonic crystal and controls the magnitude of said externalforce applied by said external force application means in accordancewith the measured value.
 3. The optical device as set forth in claim 1,furthermore comprising a heater, which heats said photonic crystal, anda temperature sensor, which measures the temperature of said photoniccrystal, and wherein the power supplied to said heater is controlled inaccordance with the temperature measured by said temperature sensor. 4.The optical device as set forth in claim 1, furthermore comprising acontainer, which houses said photonic crystal, and wherein said externalforce application means applies pressure in a fixed direction as saidexternal force to said photonic crystal that is housed inside saidcontainer.
 5. The optical device as set forth in claim 4, wherein atleast a part of the outer walls of said container is transparent or atransparent window is disposed at this part and light is input into saidphotonic crystal via said part.
 6. The optical device as set forth inclaim 1, wherein said photonic crystal has a plurality of microspheresof silica or barium titanate contained in a gel substance.
 7. Theoptical device as set forth in claim 1, wherein said photonic crystalhas a plurality of microscopic spaces formed and contained inside a gelsubstance.
 8. The optical device as set forth in claim 1, wherein saidexternal force application means is a piezoelectric element that deformsin accordance with an electrical input.
 9. The optical device as setforth in claim 1, wherein said external force application means is apressing mechanism that presses said photonic crystal in accordance witha manual input.
 10. The optical device as set forth in claim 1, whereinsaid external force application means is a hollow member, which can bedeformed in a manner such that its inner diameter changes, and saidphotonic crystal is disposed inside said hollow member.
 11. The opticaldevice as set forth in claim 4, wherein said container is formed byprocessing a semiconductor substrate.